Moon, Venus, Mercury

Only three large bodies are closer to the Sun than Earth is right now — the Moon and the planets Venus and Mercury. And all three of them are on display in the dawn sky for the next couple of mornings. Shortly after first light tomorrow, look for brilliant Venus, the “morning star,” above the Moon, with much fainter Mercury farther to the lower left of the Moon.

Earth’s average distance to the Sun — a measurement known as the astronomical unit — is about 93 million miles. Since the Moon orbits our planet, its average distance to the Sun is the same. But right now, the Moon is on the sunward side of its orbit, getting ready to cross the line between Earth and the Sun. Because of that, it’s a couple of hundred thousand miles closer to the Sun than Earth is.

Venus is the next planet inward — a bit less than three-quarters of Earth’s distance from the Sun. Venus follows the most nearly circular orbit of any planet in the solar system, so its distance from the Sun changes by only a tiny amount.

Mercury is the closest of the Sun’s planets — only about a third of Earth’s distance. But Mercury’s orbit is highly eccentric, which means its distance to the Sun varies dramatically — by about 15 million miles. Because of that, the Sun would appear obviously larger and smaller in the sky over the course of Mercury’s year.

The Sun can do some other odd things in Mercury’s sky as well, and we’ll have more about that tomorrow.

Script by Damond Benningfield, Copyright 2012

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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